Depends what works for you. Personally I just work on whatever I want to / feel like I should because if there's a set timetable you kind of look at it and dread learning the subject and therefore you don't revise as efficiently. Chemistry is by far my least favourite subject and when I was working in a grid-locked revision timetable I found I didn't revise as well when it came to chemistry and other lessons because I felt like I was forcing myself to. Now I just revise whatever I feel like I should (although I do revise for hours per night and I do make sure I revise all my subjects within the week) and it makes me enjoy chemistry more because I'm choosing to revise it and I have given myself more than an hour to really get to grips with some of the concepts.

This is just me though, it may not work for you, keep trying different things to see what works for you best